FIG. 1 very schematically shows an example of an oil drilling plant in a marine environment. This plant comprises a floating platform 1 located at the sea surface, vertically above a well 3 drilled from the sea bottom and emerging into the oil reserve to be extracted (not shown). An immersed pumping system 5 lies on the sea bottom at the level of the opening of well 3. Pumping system 5 is capable of extracting the oil from well 3. The plant further comprises a pipe 7 extending between pumping system 5 and platform 1, intended to convey to platform 1 the oil extracted from well 3. Pipe 7 is itself arranged in a protection tube 9 also extending between pumping system 5 and platform 1. The space between the inner wall of tube 9 and the outer wall of pipe 7 may contain cables 11 coupling pumping system 5 to platform 1, for example, electric cables for powering pumping system 5.
During the extraction, a mixture of water, of mud, of oil, etc. may rise from well 3, inside of the space located between oil pipe 7 and protection tube 9. The mixture may contain corrosive substances such as carbon dioxide (CO2) or hydrogen sulfide (H2S). Such substances are capable of degrading cables 11 coupling pumping system 5 to platform 1. The presence of such substances is difficult to anticipate since the latter generally come from the inside of the oil reserve, and it is not known in advance whether the reserve contains such substances and by which quantities.
There thus is a need for a device enabling to detect in situ the presence of one or of a plurality of corrosive substances inside of tube 9, for example, to be able to interrupt the extraction and replace cables 11 when an excessive concentration of such a substance is detected. It should be noted that the substances which are desired to be detected are generally in gaseous form in the pressure and temperature conditions usually encountered at the sea surface, but may be in liquid form in the pressure and temperature conditions encountered in the lower portion of tube 9.
The document entitled “Distributed Fiber Optic Gas Sensing for Harsh Environment” of Juntao Wu (www.osti.gov/scitech/servlets/purl/938805) describes a distributed gas detection device comprising an optical fiber having a portion of its sheath modified to react with the gas to be detected. The gas concentration is indirectly measured by the modification of the optical properties of the fiber. A disadvantage of this device is that the modifications of the optical properties of the fiber is irreversible, which implies that the measurement can only be performed once. Further, with such a method, it is difficult to measure the concentration of a plurality of different substances by means of a same fiber.
The document entitled “Gas detection with micro- and nano-engineered optical fibers” of W. Jin et al. (Optical Fiber Technology, Volume 19, Issue 6, Part B, December 2013, Pages 741-759), describes various technical solutions for the detection of gas. Most solutions are non-distributed, and do not enable to monitor extensive areas. The distributed solutions described in this article (section 4 of the article) are based on the direct spectroscopic analysis of a light beam interacting with the monitored gas. Such solutions require using optical fibers having complex structures, to enable part of the light propagating through the fiber to interact with the gas, and then to reintegrate the fiber, so that a reflected or back-scattered signal can be analyzed at the fiber input. More particularly, the distributed solutions described by W. Jin et al. use either optical fibers of HC-PBGF type (“Hollow-core photonic bandgap fibers”), that is, fibers having a discontinuous core, the gas to be analyzed being placed on the path of light between the successive sections of the core, or optical fibers of SCF type (“Suspended Core Fiber”), that is, fibers having a core suspended in a cavity filled with the gas to be analyzed. W. Jin et al.'s article further mentions solutions based on the principles of photo-acoustic detection. The article however does not detail a distributed photo-acoustic solution, enabling to monitor extensive areas.